1. Field of the Invention
The invention relates to an electric power steering apparatus.
2. Description of Related Art
An electric power steering apparatus (EPS) gives an assist force to a steering system by rotating a steering shaft through the use of an electric motor as a drive source. This kind of EPS in a vehicle changes its assist characteristic according to the state of travel of the vehicle by controlling a single electric motor. However, the control performed via the single motor has a limit in the setting of an assist characteristic that finely adapts to various road conditions from a flat asphalt road (flat road) to a bad or rough road such as a Belgian road or the like. Therefore, in order to further improve the degree of freedom in the assist characteristic setting, an EPS is provided with two drive sources in a steering system that extends from the steering wheel to steered wheels (e.g., Japanese Patent Application Publication No. 2005-247214 (JP-A-2005-247214)). The EPS shown in Japanese Patent Application Publication No. 2005-247214 (JP-A-2005-247214) uses, as the two drive sources, for example, a column assist type EPS (column EPS) and a pinion assist type EPS (pinion EPS), and controls the motors of the column EPS and of the pinion EPS according to the state of travel of the vehicle, so as to improve the degree of freedom in the assist characteristic setting.
However, the EPS described in Japanese Patent Application Publication No. 2005-247214 (JP-A-2005-247214) has a structure in which the column EPS and the pinion EPS are interlinked via an intermediate shaft and a joint. Therefore, when the vehicle travels on a uneven road, force that is reversely input from the road surface to the steered wheels acts on the intermediate shaft and the joint, causing a twist between the column EPS and the pinion EPS, and the vibration or rotation caused by the twist is transmitted to the steering wheel via the column EPS. Incidentally, the reverse input of force from the steered wheels is restrained by a reaction that is caused by the mechanical friction of the pinion EPS and the inertia of the electric motor. However, since the steered wheels are linked to the column EPS as well via the intermediate shaft and the joint, a large force that is reversely input, in particular, is inevitably transmitted to the steering wheel. As a result, loss of control of the steering wheel or vibration thereof occurs, so that the straight-ahead directional stability of the vehicle is lost.